1. Field of the Invention
The present invention relates to a printing system having a head, from which inks are dischargeable to form at least two different dots having different characteristics, for example, at least two different dots having different densities per unit area or at least two different dots having different hues, on an object, and recording a multi-tone image by a distribution of the at least two different dots. The present invention also pertains to an image recording method for recording a multi-tone image in such a printing system and a recording medium for storing a program to cause a computer to realize the image recording method.
2. Description of the Related Art
Color printers, in which a plurality of color inks are discharged from a head, are widely used as an output device of a computer that records a multi-color, multi-tone image processed by the computer. Several methods are applicable to print a multi-color, multi-tone image with three color inks, cyan, magenta, and yellow (CMY). One method is a technique adopted in the conventional printers. This technique expresses the tone of a printed image by the density of dots (frequency of appearance of dots per unit area) while fixing the size of dots formed on a sheet of paper by a spout of ink. Another method adjusts the diameter of dots formed on a sheet of paper, in order to vary the density of dots per unit area. Although the advanced processing of the head for forming ink particles has been improving the density of dots formable per predetermined length or the variable range of the dot diameter, the printers have only the limited printing density or the resolution to 300 through 720 dpi and the limited particle diameter to several ten microns. The resolution of printers is significantly lower than the resolution of silver photography, which has reached several thousand dpi on the film.
Dots are sparsely formed in an area of low image density, that is, in an area of low density of dots to be printed. This increases the degree of granularity and makes the dots conspicuous. A printing system and a method utilizing inks of different densities have been proposed to improve the printing quality. The proposed technique utilizes a high-density ink and a low-density ink for a certain color and regulates discharge of these inks, thereby realizing print with an excellent tone expression. By way of example, a method of and an apparatus for recording a multi-tone image are disclosed in JAPANESE PATENT LAYING-OPEN GAZETTE No. 61-108254. The disclosed technique provides a head for forming deep dots and light dots for a certain color and regulates the number and overlap of deep dots and light dots formed in a predetermined dot matrix according to input density information of an image, so as to record a multi-tone image.
The proposed printing system utilizing inks of different densities, however, does not give any specific idea on the allocation of high-density ink and low-density ink to the input tone signals of an original image. Inks of different densities are simply allocated in the order of densities to the input tone signals of the image (for example, FIG. 9 in JAPANESE PATENT LAYING-OPEN GAZETTE No. 2-215541).
The on/off state of two different dots may be determined independently. This is attained, for example, by using two completely different dither matrices or by shifting either one or both of the horizontal and vertical positions in one dither matrix. These conventional methods ensure good dispersibility of the respective dots, but dot not realize the optimum dispersibility of these dots as a whole, which results in output images having a high degree of granularity and unevenness. Similar problems arise in any dots having different characteristics other than the dots having different densities, for example, at least two different dots formed by inks of different hues and at least two different dots having different sizes.